1. Field of the Invention
The invention relates to a circuit arrangement for controlling a d.c. motor which can be driven in two directions, which arrangement comprises control means having a reference potential input and a control input, by means of which the speed of the motor is controllable.
2. Description of the Related Art
Such a circuit arrangement is known, for example, from the cassette deck Alpine type GR-H.
In this known cassette deck the capstan motor is operated in two directions of rotation, one direction of rotation serving for the tape transport of the magnetic tape and the other direction being used for servo functions. The control means are realized, for example, by means of a three-point controller, which is available from, for example, Matsushita as the integrated circuit AN6605-N. For the direction reversal an H-bridge is used, for example as revealed in the book "Elektrische Kleinmotoren", Helmut Mugschalla, Expert-Verlag, FIG. 8.15. In this known bridge circuit a transistor is disposed between the motor and the supply voltage and a transistor is disposed between the motor and the control input of the control means in both directions of rotation of the motor. This has the disadvantage that, for example in the case of bipolar transistors the collector-emitter voltages of the conducting transistors influence the motor voltage and hence the speed, because these collector-emitter voltages cannot be measured and corrected by the control means. These collector-emitter voltages are dependent both on the collector current and on the temperature. A further disadvantage of this known bridge circuit is that the base current of the transistor arranged between the motor and the control input of the control means flows into the control input of the control means but does not flow through the motor. As a result of this, fluctuations of the base current of this transistor influence the control means without the current through the motor having changed.